Early attempts to increase fluid flow area around the wellbore of a subterranean production well, such as an oil and/or gas production well, used devices and materials such as nitroglycerin, dynamite, or other such high energy materials to produce an explosive event that would create flow area at desired locations. These early methods had only limited success. A presentation of Cuderman's work at the Society of Petroleum Engineers (SPE) conference in Pittsburgh, Pa. on May 16-18, 1982, confirmed the existence of a preferred multiple fracture regime under certain firing conditions. Cuderman demonstrated that pressure rise time was an important factor for increasing near wellbore permeability. FIG. 1 illustrates the findings of Cuderman in chart form. Cuderman described three fracture regimes of underground formations. Based on this information, other technologies were developed.
More specifically, Cuderman demonstrated the existence of a hydraulic fracture regime, an explosive fracture regime, and an intermediate multiple fracture regime (see SPE/DOE 10845, “Multiple Fracturing Experiment-Propellant in Borehole Considerations” by Jerry F. Cuderman). The hydraulic fracture regime is characterized by a slow pressure rise that occurs when fluid flows to the point of least resistance. To create formation characteristics in the multiple fracture regime, a more rapid pressure rise is required. Pressure developed in the hydraulic fracture regime flows to the point of least resistance, usually generating a bidirectional, two-dimensional fracture. In contrast, the explosive fracture regime is created when a very rapid pressure rise of short duration is produced. Frequently, the explosive fracture regime causes formation damage and rubblization, damaging and sealing off some of the pore space. This results in an undesirable loss of porosity.
A number of inventors have attempted to use propellants in wells to achieve various goals; some of these are listed below in Table 1.
TABLE 1InventorPatent No.Issue DateSnider et al.5,775,426Jul. 7, 1998Passamaneck5,295,545Mar. 22, 1949Hill et al.4,683,943Aug. 4, 1987Hill et al.4,633,951Jan. 6, 1987Ford et al.4,391,337Jul. 5, 1983Hane et al.4,329,925May 18, 1982Godfrey et al.4,039,030Aug. 2, 1977Mohaupt3,313,234Jan. 13, 1958
Each of these techniques has issues with wellbore conditions, explosive propellants, and/or minimal effective stimulation due to lack of or loss of energy.
Snider '426 describes a method of surrounding at least one perforating shaped charge with a sleeve of propellant, and uses the perforating charge blow a hole through the propellant and ignite it. The propellant gas is then used to create fractures in the near wellbore. A system is used that utilizes a shaped charge, or many shaped charges, to ignite the propellant sleeve. This type of ignition makes it difficult to predictably reproduce the event. Shaped charges are configured to blow through pipe and cement, thereby creating a tunnel for fluid flow. The entry hole size varies widely, e.g., from 0.19″ to 1.10″ and from 1 shot per foot up to 18 shots per foot (or more). This does not allow for a predictable, consistent amount of propellant surface area to be ignited. The propellant of Snider is broken into a random number of pieces, resulting in unpredictable pressure rise and propellant flow results.
Passamaneck '545 describes a method of externally igniting an external portion of a propellant charge to burn inwardly, thus yielding a more predictable ignition of the external propellant surface. Although the ignition system is predictable, the fluid in the wellbore keeps the propellant from reaching the critical pressure rise time needed to achieve a multiple fracture regime because of fluid leaching into the propellant. Much of the energy required for formation treatment is lost to the well fluid that inhibits the burn.
Hill '943 and '951 uses a compressible fracturing fluid to carry the propant into the fractures, causing hydraulic fracturing due to the energy stored in the “compressible” fluid.
Ford '337 describes positioning propellant having an abrasive material directly adjacent a shaped charge that is subsequently ignited. The shaped charge ignites the propellant gas and propels the abrasive material, thereby enlarging the perforation holes and extending fractures. The extended fractures are propped open by the abrasive material.
Hane '925 describes a method of utilizing multiple explosive charges in an effort to rubblize and fracture the formation.
Godfrey '030 describes a method of igniting a propellant tens of feet above a high explosive disposed adjacent to the pay zone, with the high explosive and the propellant being suspended in fracturing fluid. Godfrey's technique attempts to extend the duration of the shock wave caused by the high explosive.
Mohaupt '234 describes a method of igniting a propellant-type explosive that is dispersed into the wellbore liquid. This allows it to be ignited and reignited to cause pressure oscillations.
Subterranean wells often have a restricted flow area near the wellbore. Examples of such wells can include oil and/or gas producing wells, injection wells, storage wells, brine or water production wells, and disposal wells. The restricted flow area can be caused by the overburden exerting excessive compression on the formation near the wellbore, or by man-made damage near the wellbore, e.g., during drilling operations. For example, fluids or materials introduced into the wellbore can restrict permeability, reducing fluid communication and decreasing flow capacity to the pay zone. Certain wells have pay zones that cannot be effectively produced without some type of stimulation. Such wells are usually “tight” and require that additional flow area be opened to enable the wells to become commercially viable.
The technologies described in the documents above each attempt to create multiple fractures near the wellbore or open fractures near the wellbore prior to a hydraulic fracture, thereby increasing formation permeability and enhanced flow characteristics near the wellbore. Unfortunately, they each possess certain limitations. For example, none of them utilize a predictable internal ignition system to enable them to reach a critical pressure rise time necessary to enter into the multiple fracture regime and to provide sufficient gas volume to be able to extend the multiple fractures sufficiently far into the formation while protecting the propellant from the fluid in the wellbore.
What is needed is a method and apparatus utilizing an internal ignition in combination with a propellant charge that creates fractures into the wellbore in the multiple fracture regime, and extends these fractures further into the subterranean formation, thereby providing for an extended radial flow area that enhances well capacity and production capabilities.